1. Field of the Invention
The present invention relates to a compound semiconductor device having a field plate electrode.
2. Description of the Related Art
A compound semiconductor device using a group-III nitride semiconductor is used for a high-voltage power device and the like. Atypical group-III nitride semiconductor is expressed as AlxInyGa1-x-yN (0</=x</=1, 0</=y</=1, 0</=x+y</=1), and includes gallium nitride (GaN), aluminum nitride (AlN), and indium nitride (InN). For example, for a high-electron-mobility transistor (HEMT), a hetero junction surface is formed in an interface between a carrier travel layer and a carrier supply layer which are made of the group-III nitride semiconductor with different bandgap energies from each other. In the carrier travel layer near the hetero junction surface, a two dimensional carrier gas layer is formed as a current path (channel).
A bias electric field generated when a voltage is applied between a drain electrode and a source electrode of a compound semiconductor device concentrates in an end portion of the gate electrode on the side of the drain electrode (hereinafter, referred to as a drain-side end portion). By alleviating the concentration of the bias electric field in the drain-side end portion of the gate electrode, the voltage resistance of the compound semiconductor device can be improved. For example, a method is proposed for easing electric field concentration in the drain-side end portion of the gate electrode by arranging a field plate electrode.
In forming electrodes such as a drain electrode on an insulation film formed on a group-III nitride semiconductor layer, opening portions are made in the insulation film. Then, in order to connect the respective electrodes and the group-III nitride semiconductor layer, the electrodes are located in the opening portions respectively. At this time, an area of each of the electrodes is formed wider than the corresponding opening portion to secure process margin in the semiconductor manufacturing process. As a result, an area where the each of the electrodes and the group-III nitride semiconductor layer face each other across the insulation film (hereinafter referred to as a flange portion) is formed. The flange portion functions as a field plate electrode.
It is known that a field plate electrode electrically connected to a drain electrode worsens current collapse. Therefore, in order to decrease the function of a flange portion of a drain electrode as a field plate electrode, a film thickness of an insulation film on a group-III nitride semiconductor layer needed to be increased. In this case, however, a problem arose that an effect to alleviate electric field concentration due to a conventional field plate electrode was reduced.